Various types of printed wiring boards are employed for mounting electronic components in liquid crystal devices of personal computers and cellular phones, and in printers. These printed wiring boards are produced by a series of steps in which at least one major surface of a flexible insulating film such as a polyimide film or a rigid insulating plate such as a glass epoxy plate is overlaid with a conductive metal layer, the conductive metal layer is coated with a photosensitive resin, the photosensitive resin is photoexposed and patterned in desired configuration, the conductive metal layer is etched using the photopatterned resin as a mask to create a wiring pattern of the conductive metal, a solder resist is applied on the wiring pattern except terminals (inner leads) that make contact with electronic components mounted thereon and terminals (outer leads) that are in communication with the inner leads and are connected with the outside, the solder resist is cured, and the inner and outer leads are plated with tin or the like.
On the thus-prepared printed wiring board for mounting electronic components, a semiconductor chip is placed such that the bump electrodes formed on the chip are contacted with the inner leads, and the bump electrodes and the inner leads are electrically connected using a bonding tool. Mounting of an electronic component on the printed wiring board is thus accomplished.
In recent years, semiconductor devices are used in which semiconductor chips such as ICs and LSIs and passive components such as capacitors and resistors are mounted on one printed wiring board (see, for example, JP-A-2003-124601).
Miniaturization and weight reduction of electronic equipment have created demands for higher mounting density of electronic components. In the semiconductor devices, increasing the mounting density results in intersection of wirings among the mounted components, and therefore the electrical connection of these components by wirings can not be accomplished. To overcome such problems, double-sided printed wiring boards have been employed such as 2 metal TAB tapes and 2 metal FPC in which wiring patterns are provided on both surfaces of an insulating layer and the patterns on the both surfaces are connected through vias in a highway crossing configuration.
The double-sided printed wiring boards, however, have high costs because wiring patterns must be formed on one surface and then the other. Therefore, it has been worked out for increasing the mounting density that a wiring pattern is created only on one surface of the insulating layer, and a component is mounted on the reverse surface of the wiring pattern-formed surface through a via when the connections will cross one another.
In an etching treatment for forming a wiring pattern or a tin or gold plating treatment, however, an etching or a plating solution or a rinse liquid for removing the treatment solution leaks in a gap between the filled via and the insulating layer. The acidic plating solution or the like remained in the gap can leak out therefrom in the post processes, or such residual liquid evaporates and expands with heat applied when bonding a semiconductor chip or applied in the other process, resulting in a small explosion.
The present invention solves the aforesaid problems of the background art. It is therefore an object of the invention to provide a printed wiring board for mounting electronic components which includes an insulating layer and a wiring pattern formed on one surface of the insulating layer and in which an electronic component can be mounted on the reverse surface of the wiring pattern-formed surface through a filled via, wherein a wet processing liquid such as an etching or a plating solution is prevented from leaking in a gap between the filled via and the insulating layer. The invention also provides a production process for making the printed wiring board as well as a semiconductor device fabricated using the printed wiring board.